This invention relates to olefin polymerization catalysts and more particularly relates to an improved supported propylene polymerization catalyst.
Polymerization of alpha-olefins, such as ethylene, propylene and butene-1, to normally-solid, substantially crystalline polymers using catalyst compositions comprising transition metal and organoaluminum compounds is well known. Although many transition metal compounds have been disclosed as useful in such polymerizations, including salts of titanium, vanadium and zirconium, compounds of titanium predominate with tetravalent titanium (Ti(IV)) compounds typically proving most useful in ethylene polymerization and trivalent titanium (Ti(III)) compounds most useful in polymerization of propylene and higher alpha-olefins. Various forms of titanium components have been disclosed in both Ti(IV)- and Ti(III)-based compositions including a so-called "supported" titanium component. Supported titanium catalyst components have been disclosed in which titanium (either Ti(III) or Ti(IV)) is placed onto a metal oxide or metal halide support. One view of an advantage of supported titanium catalyst components is that in such components catalytically active titanium-containing sites are dispersed throughout the supported catalyst composition yielding more efficient use of the titanium content and resulting in higher overall catalytic activity, especially if based on titanium content.
Supported titanium catalyst components have been found most useful in ethylene polymerization such as described in U.S. Pat. No. 3,901,863. Until recently, however, supported titanium catalyst components have not been used substantially in commercial polymerization of propylene or higher alpha-olefins due to coproduction of high levels of noncrystalline or amorphous polymeric products. Production of such noncrystalline, amorphous products especially is detrimental in polymerization processes in which such noncrystalline products are not removed by solvent extraction.
Examples of nonsolvent processes are gas-phase and bulk processes. In gas-phase polymerization, solid polymer is formed directly on contact of gaseous monomer with a catalyst; while in a bulk process, polymer is formed in a liquid monomer medium. A combination of such processes is possible where polymerization occurs in two or more stages. In either process solid polymer advantageously is not treated further with a hydrocarbon solvent to remove noncrystalline material. Even in a slurry process in which noncrystalline material can be removed, production of such noncrystalline material usually is minimized because of its lower economic value.
Supported olefin polymerization catalysts have been disclosed in which crystallinity-promoting components are incorporated. Such components can be electron donor compounds which are associated with titanium-containing supported catalyst complexes. Further, preparations of supported olefin polymerization catalysts have been disclosed in which the supported catalyst material is comminuted, such as by ball-milling, in order to increase catalyst activity. An example of such catalyst material is described in U.S. Pat. No. 4,277,370 incorporated by reference herein. However, it has been found that while comminuting such a catalyst component can increase activity, presumably by exposing more active sites, such a comminuted catalyst also can yield increased noncrystalline polymer products as evidenced by increased solubles and extractables.
Olefin polymerization catalysts having higher activity and yielding fewer undesirable by-products are always desirable. Many supported titanium-containing catalyst components which have been comminuted, such as by ball-milling, have been found to be active; however, such a comminuted supported catalyst component which also yields low amounts of noncrystalline polymeric by-products would be very desirable. A method to produce such a catalyst would be very useful in the olefin polymerization catalyst art.
U.S. patent application Ser. Nos. 378,121, 378,404 and 378,406 all filed May 14, 1982 and now U.S. Pat. Nos. 4,431,570, 4,431,571 and 4,431,572, respectively, and European Published Application No. 95,290, all incorporated by reference herein, describe catalyst systems in which comminuted supported titanium-containing catalyst component is retreated with a mixture containing a titanium halide, at least one organic acid ester and, optionally, a chlorocarbon or a haloalkylchlorosilane. The invention described herein is an improvement over such catalysts in that the organic acid esters used in the retreatment step are specified as a mixture of particular esters with identifiable properties. The catalysts prepared according to this invention are shown to be superior to similar catalysts prepared without using the specific mixed esters described herein.
In another aspect of this invention, it has been found that catalysts prepared according to this invention are superior when tested at commercial slurry polymerization conditions. Many laboratory slurry polymerization tests are started by injecting catalyst into a relatively "cool" reaction medium. However, in a commercial slurry olefin polymerization reaction, catalyst is injected continuously into a "hot" reaction medium. It has been observed that catalysts of this invention prepared using a mixed ester retreatment show greater activity and better stereospecificity to similar catalysts prepared using a single ester retreatment. Thus, in "hot" start conditions the catalysts of this invention have been shown superior at commercial conditions.